The present invention relates to a projection exposure apparatus. To describe it in more detail, the present invention relates to a projection exposure apparatus that can effectively prevent thermal damages and improve the depth of focus and the resolution.
In order to enhance the integration densities and increase operational speeds of a variety of solid-state devices such as LSI chips, miniaturization of circuit patterns is under way. At the present time, the reduction-type projection exposure apparatus, which has an excellent mass-productivity characteristic and displays superior resolution, is widely used in the creation of circuit patterns.
The resolution of the reduction-type projection exposure apparatus is proportional to the exposure wavelength but inversely proportional to the numerical apertures (NA). The depth of focus of the reduction-type projection exposure apparatus is, on the other hand, proportional to the exposure wavelength but inversely proportional to the square of the NA. Accordingly, increasing the NA and shortening the wavelength are effective techniques for improving the resolution. In the case of the conventional reduction-type projection exposure apparatus, these techniques have been used for achieving the enhanced resolution.
Increasing the NA and shortening the wavelength, however, inevitably make the depth of focus extremely short, giving rise to a problem described below.
As the integration density of an LSI chip becomes higher, dimensions of a variety of circuit patterns such as interconnections are getting miniaturized. The structure of a semiconductor device such as DRAM goes three-dimensional, introducing more and more steps and a higher and higher degree of unevenness on its surfaces. When projecting a mask pattern on the surface of a semiconductor substrate using a reduction-type projection exposure apparatus with a reduced depth of focus described above, therefore, some of the lower or upper portions of the steps or uneven profiles on the surface of the semiconductor substrate inevitably protrude out off the rang of the reduced depth of focus. As a result, it becomes difficult to create fine patterns over the entire surface of an LSI chip with a high degree of accuracy.
With the conventional techniques of increasing the NA and shortening the wavelength as described above, it is thus impossible to satisfy the needs for high resolution and a sufficient depth of focus simultaneously. As a result, some means for solving the problems are strongly demanded.
In order to achieve high resolution while, at the same time, retaining the required depth of focus, the present inventor discovered, at an early time, a so-called pupil filtering technique of installing a spatial filter, which has a special complex amplitude-transmission distribution at the pupil position of the projection lens, for improving the depth of focus and the resolution of the reduction-type projection exposure apparatus described above. Extended abstracts of this pupil filtering technique are explained on page 534 of the Japanese Society of Applied Physics and Related Societies, Issue No. 2 presented to the 38th Spring Meeting held in the year of 1991.
The outline of the pupil filtering technique is shown in FIG. 2. Reference numeral 1 shown in FIG. 2 denotes a reticle having a pattern to be created whereas reference numeral 2 is the projection lens of a reduction-type projection exposure apparatus. Reference numeral 3 is a spatial filter. The known reduction-type projection exposure apparatus prints the image of a predetermined pattern created on the reticle 1, onto a resist film on the substrate through the projection lens 2. Note that the resist film itself is not shown in the figure. The spatial filter 3 is fixed at a pupil position 4 or at an aperture step which determines the numerical aperture of the projection lens 1. The spatial filter 3 has a complex amplitude-transmission distribution expressed by Equation (1) as follows: EQU T(r)=cos (2.pi..multidot..beta..multidot.r.sup.2 -.theta./2).times.circ(r) (1)
where .beta. and .theta. are appropriate constants.
Equation (1) expresses the complex amplitude-transmission distribution as a function of radial-direction coordinate r. In this case, the radial-direction coordinate r is normalized by a maximum radius of the pupil or the aperture.
A resultant image can thereby be obtained by adding the amplitudes of two images A and B, to be created at two different positions +.beta. and -.beta. in the optical direction, with the amplitudes treated as vectors having a difference in phase corresponding to the constant .theta.. As a result, a deep depth of focus and high resolution can be retained even if the NA is increased and, on top of that, the wavelength is shortened.
However, the pupil plane conjugates with a secondary illumination plane (effective source plane) of the reduction-type projection exposure apparatus. Accordingly, the illumination-intensity distribution of the secondary illumination source is redisplayed as it is. The secondary illumination source, which serves as the exit plane of an ordinary fly-eye lens, is a collection of point illumination sources having a sharp peak intensity focused by an effect of the fly-eye lens. Therefore, the light is also concentrated into spots on the lens pupil, resulting in a locally increased illumination intensity. An amplitude attenuating filter made of a photo-absorption material fixed on such a pupil plane dissipates heat because the amplitude attenuating filter absorbs light. Accordingly, the amplitude attenuating filter is thermally damaged, giving rise to a problem that the amplitude attenuating filter can no longer be used. In addition, the dissipated heat causes the substrate of the filter or the projection lens to partially expand. The expansion results in a distorted shape or, change in the index of refraction.
Accordingly, the optical characteristic of the reduction-type projection exposure apparatus deteriorates. As a result, there is the possibility of difficulty in creating a fine pattern with a high degree of accuracy.
It should be noted that patent application related to the present invention are those with an Ser. No. 07/622,606 filed on December 5, 1990, Ser. No. 07/765,060 filed on Sep. 24, 1991 and Ser. No. 07/846,158 filed on Mar. 5, 1992.